docs: def facets_by_halfspace(self):
          """
          Returns a list of vertex indices for each halfspace
          e.g: facets_by_halfspace[0] is the list of indices ov vertices
          incident to halfspace 0
          """
          if self._fbh_out is None:
              output = qhalf('FN', self.halfspaces, self.interior_point)
              facets = []
              for l in output[1:]:
                  facets.append([int(i) for i in l.split()[1:]])
              self._fbh_out = facets
          return self._fbh_out